1. Field of the Invention
The invention relates to a magneto-optical waveguide device for the conversion of modes of propagation of the device, which device comprises a magneto-optical layer in which the radiation-modes propagate and which is magnetized externally, a substrate carrying the magneto-optical layer, and an optically anisotropic structure which extends parallel to the plane of said layer for phase adaptation of the radiation-modes in the case of total reflection from the boundary surfaces of the magneto-optical layer.
2. Description of the Prior Art
A waveguide device of this type is generally known. It serves to convert the various radiation-modes in the magneto-optical layer into each other (mode conversion). The fundamental modes are pure or at least substantially transverse magnetic (TM) and transverse electric (TE) waves, whose amplitudes are mathematically related through a scattering matrix. The radiation-modes are coupled by total reflection from the boundary surface of the magneto-optical layer and in the magneto-optical layer itself, which layer is magnetized in the direction of propagation of the modes. In order to obtain a satisfactory conversion of the wave modes into each other, a substantial number of reflections is required. However, a cumulative superposition demands specific phase relationships of the waves between the individual reflections, i.e. a substantially equal phase velocity of the TM waves and TE waves in the waveguide structure, which is also referred to as phase adaptation. Normally, this is not achieved if the waveguide is a purely gyrotropic dielectric film. In order to obtain phase adaptation, an additional optical anisotropy is required, for example an anisotropic top layer which affects the phase discontinuities of the modes in the case of total reflection from the boundary surfaces of the magneto-optical layer.
From the paper by S. T. Kirsch, J.Appl.Phys.52 (5), May 1981, pages 3190 to 3198, it is known to employ a single-crystal layer of lithium niobate (LiNbO.sub.3) for such a optically anisotropic top layer. Such layers are deposited on the magneto-optical layer, which may for example consist of yttrium-iron garnet (YIG). However, because of the difference in lattice structure of the magneto-optical layer and top layer an intermediate layer, for example a selenium layer, must be interposed.
A waveguide device of this type therefore has a layer structure which is comparatively intricate and difficult to manufacture. As a result of the presence of the intermediate layer such devices are not suitable for industrial fabrication.